A fuel cell of a fuel cell unit of a fuel cell system uses electrochemical conversion of a hydrogen-containing (H, H2) fuel to water, using oxygen (0, 02) to generate electrical energy. For this purpose, the fuel cell contains at least one so-called membrane electrode assembly (MEA) as a core component, which is a structure of an ion-conducting, or proton-conducting membrane and electrodes situated on both sides of the membrane, an anode electrode, and a cathode electrode. In addition, gas diffusion layers (GDL) may be situated on both sides of the membrane electrode assembly on the sides of the electrodes facing away from the membrane.
In general, the fuel cell is formed by a plurality of membrane electrode assemblies arranged in a stack, their electrical powers being combined during operation of the fuel cell. Bipolar plates, also known as flow field plates or separator plates, are usually situated between the individual membrane electrode assemblies, which ensure supply of operating media, known as reactants, to the membrane electrode assemblies, i.e., supply to the individual cells of the fuel cell, also usually serving as coolants. In addition, the bipolar plates ensure an electrically conductive electrical connection to the adjacent membrane electrode assemblies.
During operation of the individual cells of the fuel cell (individual cell: membrane electrode assembly and an associated anode space delimited by a bipolar plate and an associated cathode space delimited by a second bipolar plate), the fuel, a so-called anode operating medium, is fed to the anode electrodes via a flow field of the bipolar plate, open on the anode side, where an electrochemical oxidation of H2 to 2H+ takes place (H2=>2H++2e−) with the release of electrons (2e−). Water-bound or water-free transport of the formed protons (H+), from the anode electrodes ((complex) anode of the fuel cell), in the anode spaces of the individual cells to the cathode electrodes ((complex) cathode of the fuel cell) in the cathode spaces of the individual cells takes place through the membranes or electrolytes of the membrane electrode assemblies, which gas-tightly separate and electrically insulate the respective reaction spaces (anode space-cathode space pairs of the individual cells).
The electrons provided on the anode are conducted to the cathode via an electrical conductor and an electrical consumer (electric traction motor, AC unit, etc.). An oxygen-containing cathode operating medium is fed to the cathode electrodes of the cathode via a flow field of the bipolar plates open on the cathode side, a reduction from O2 to 2O2− taking place with the reception of electrons (½O2+2e−=>O2−). At the same time, the oxygen anions (O2−) formed at the cathode electrodes react with the protons transported through the membranes or electrolytes, with the formation of water (O2−+2H−=>H2O)
In order to supply a fuel cell stack, henceforth referred to mainly as fuel cell, with the operating media, the fuel cell has on the one hand, an anode supply and, on the other hand, a cathode supply. The anode supply has an anode supply path for feeding the anode operating medium into the anode spaces of the fuel cell and an anode exhaust gas path for removing an anode exhaust gas from the anode spaces. Similarly, the cathode supply has a cathode supply path for feeding the cathode operating medium into the cathode spaces of the fuel cell and a cathode exhaust gas path for removing a cathode exhaust gas from the cathode spaces.
For operating the fuel cell system, oxygen must be provided as cathode operating medium, mostly in the form of ambient air. Depending on the mode of operation of the fuel cell system, the air must be supplied compressed to a certain fluid pressure above the ambient air pressure (approximately 1 bar and less) and at a certain air mass flow rate. In particular, at comparatively high air masses and comparatively high fluid pressures, this is no longer feasible using a single one-stage fluid pumping device. In modern fuel cell units, an electrically driven turbocharger, known as electric turbocharger, is used. The electric turbocharger is the largest parasitic consumer in the fuel cell system.
Furthermore, the cathode exhaust gas of the fuel cell system has a certain energy content due to its temperature level and its pressure level. This energy may be recovered using a turbine, for example. Modern passenger car systems have a power class of 70 kW to 100 kW. Accordingly, components and modules are available for this power class. Scaling a fuel cell system for passenger cars or other vehicles to powers of 120 kW to 180 kW, for example, and more [is] presently not known. Furthermore, a plurality of exhaust gas turbochargers or electric turbochargers and/or suppliers for the same, are not available for fuel cell units as they are for other motors such as internal combustion engines.